The present invention relates to a method for producing an optically recorded disk such as a read-only optical disk, a write-once type optical disk, a read-write optical disk, or the like.
In an optical disk such as a video disk or the like, the radial interval between adjacent tracks of pits or guide grooves concentrically and spirally formed on a recording surface of the disk is a minute value of about 1.6 .mu.m. When such an optical disk is mounted on a turntable of a reproducing apparatus, a spindle of the turntable is fitted into the center hole of the disk. Accordingly, the positioning of the optical disk relative to the spindle is extremely important in view of the accuracy of rotation of the disk. Further, if the displacement of the center of the optical disk is large, accurate tracking by the laser beam of the optical pickup may be impossible, even though a tracking servo is provided to correct tracking errors because the range of the servo is exceeded. Accordingly, when a center hole of an optical disk is formed, a highly accurate process is required so as to make as small as possible the amount of displacement (eccentricity) of the center point of the center hole from the true center point of the recorded track.
Conventionally, formation of the center hole of an optical disk has been performed by cutting a substrate of a transparent circular plate prepared by injection molding of poly methyl methacrylate (PMMA) resin, polycarbonate PC resin or the like. The center of the substrate (the center of the spiral of guide grooves) is determined using an optical instrument, and the substrate is bored while being firmly fixed. In the cutting process, however, there has been a problem in that the cutting machine is large in size and the substrate tends to suffer damage on its surface.
Further, alternatively, a metal mold which can form a center hole of a substrate has sometimes been used so as to directly mold a substrate having a center hole while the substrate is being formed through injection molding with resin. In this case, however, there is a problem in that the accuracy is so low that a center hole with acceptable circularity and concentricity cannot .be obtained.
In order to solve the foregoing problems, there has been proposed an optical disk constructed such that, as shown in FIG. 1, a metal hub 25 is fixed at a center portion of a substrate 22 provided with rows of pits or guide grooves formed concentrically or spirally, and a center hole 23 is formed in the metal hub 25 at a center portion thereof. In such an optical disk, because the center hole 23 is formed in the metal hub 25, a perfectly circular center hole can be easily obtained. Accordingly, not only can the eccentricity of the optical disk be reduced, but it becomes possible to use a magnetic clamping technique whereby a magnet is used in a clamping mechanism in the reproducing apparatus for clamping the optical disk via the metal hub 25 to thereby fix the optical disk at the playing position.
However, there is a problem in that the metal hub 25 and the substrate 22 consist of respectively, an inorganic material, and an organic material, and therefore, not only it is difficult to adhere the former to the latter, but it is also impossible to obtain a wide junction area between the metal hub 25 and the substrate 22. Hence, it is difficult for the metal hub 25 to adequately support the substrate 22. Further, the metal hub 25 has a tendency to separate from the substrate when the optical disk is rotating.
Further, in order to join the metal hub 25 and the substrate 22, there has been considered a joining method using ultrasonic welding in which a resin layer is formed between the metal hub 25 and the substrate 22, and ultrasonic waves are applied to the metal hub 25 and the substrate 22 from both the metal hub side and the substrate side to thereby join them to each other. In this method, however, only thermoplastic resin can be used as the substrate 22 and the resin layer. Accordingly, there is a problem in that the range of selection of the material of the substrate 22 is narrow. There is a further problem in that a horn portion of the apparatus generating the ultrasonic waves must be placed in contact with the metal hub 25 and the substrate 22 after a positioning step in which the respective centers of the metal hub 25 and the substrate 22 are caused to coincide with each other. As a result, vibration due to the contact of the horn portion with the metal hub 25 and the substrate 22 is unavoidably generated, making it difficult to perform accurate centering between the metal hub 25 and the substrate 22.